My idea of the interaction is a grey heart icon which, upon clicking, would set the tag for that member and change the heart red (instagram style). I guess I would need some jquery/ajax magic to make that happen wouldn't I? Any pointers or things to look out for?

The above is an example using blog posts and assumes the tags you add to the member are blog post slugs. You haven't specified what type of content you're using so you may need to modify the above to make it work for your use case.